1. Field of the Invention
The present invention relates to a method of and an apparatus for extracting a contour of an image, and or more particularly extracting a contour running at the boundary between pixels.
2. Description of the Related Art
In prepress process, some image parts are enlarged or reduced, rotated, and laid out to constitute one-page-image before printing plates of the one-page-image are produced, The one-page-image was previously manually prepared. Recent advances in computer technology have allowed the preparation of an one-page-image with the aid of computers.
In preparation of an one-page-image, contour tracing of image parts are performed to extract data prior to processing the image parts. After the contour extraction, the portions of the image parts defined by the contours are processed in various ways: for example, a fine conversions including rotation, enlargement, and reduction; coloring the image portion; and logical operation between the contours such as logical product (AND), logical sum (OR), exclusive OR, and inversion.
A method of contour tracing is disclosed in UK Patent GB 2,201,562A, to Miyatake et. al., published on Sep. 1, 1988. The contour in this method is defined by segments drawn between boundaries of pixels. This method includes the steps of: comparing contour lines of an image on plural scanning lines with predetermined basic contour patterns, which are illustrated in FIG. 2 of the patent, to thereby identify the basic contour patterns included in the contour lines; and linking the identified basic contour patterns for all the scanning lines on a screen page to make the contour of the image.
Since basic contour patterns are initially searched on all of the scanning lines in the prior art method mentioned above, a device for other processing such as affine conversion should suspend its operation and stand by during the search. Thus the overall processing for the image requires an undesirably long time.